Boiler Temp probe- Help.

I have a Ariston a/23 mffi boiler, that has started shutdown with an overheat warning light. This mainly occurs when the hot water is run for a while (like for a bath).Once reset it seems to runs ok.
I've noticed that the temp display on front of the boiler only work intermittently.
I 'think' that it probably the temp probe that is faulty, so tested this with a dmm and showed high/infinite resistance.
I would be greatful if someone more knowledgeable could reasure me that this is the faulty part.
Also will replacing this will require the system to be drained first.
Thanks Chris D.
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